The Friedel–Crafts reaction (1), named after its respective French and American pioneers, has long been used to attach alkyl groups to aromatic rings. This reaction is an electrophilic aromatic substitution. Electrophiles are electron-poor species, whereas nucleophiles are electron-rich species. Electron-rich aromatic molecules such as benzene can use their surplus of electrons to react with electron-poor, positively charged species, enabling benzene to exchange one of its hydrogen atoms for the electrophilic group to make a new, substituted benzene. Because "like species repel," electron-rich species should not react with other electron-rich species, so benzene should never undergo direct nucleophilic alkylation.
With the recent soaring production of natural gas, the use of methane and other light hydrocarbon feedstocks as starting materials in synthetic transformations is becoming increasingly economically attractive, although it remains chemically challenging. We report the development of photocatalytic C–H amination, alkylation, and arylation of methane, ethane, and higher alkanes under visible light irradiation at ambient temperature. High catalytic efficiency (turnover numbers up to 2900 for methane and 9700 for ethane) and selectivity were achieved using abundant, inexpensive cerium salts as photocatalysts. Ligand-to-metal charge transfer excitation generated alkoxy radicals from simple alcohols that in turn acted as hydrogen atom transfer catalysts. The mixed-phase gas/liquid reaction was adapted to continuous flow, enabling the efficient use of gaseous feedstocks in scalable photocatalytic transformations.
The C–H bonds of methane are generally more kinetically inert than those of other hydrocarbons, reaction solvents, and methane functionalization products. Thus, developing strategies to achieve selective functionalization of CH4 remains a major challenge. Here, we report transition metal–catalyzed C–H borylation of methane with bis-pinacolborane (B2pin2) in cyclohexane solvent at 150 C under 2800 to 3500 kilopascals of methane pressure. Formation of mono- versus diborylated methane is tunable as a function of catalyst, with the ruthenium complex providing the highest ratio of CH3Bpin to CH2(Bpin)2. Despite the high relative concentration of cyclohexane, minimal quantities of borylated cyclohexane products are observed.
The selective oxidation of methane, the primary component of natural gas, remains an important challenge in catalysis. We used colloidal gold-palladium nanoparticles, rather than the same nanoparticles supported on titanium oxide, to oxidize methane to methanol with high selectivity (92%) in aqueous solution at mild temperatures. Then, using isotopically labeled oxygen (O2) as an oxidant in the presence of hydrogen peroxide (H2O2), we demonstrated that the resulting methanol incorporated a substantial fraction (70%) of gas-phase O2. More oxygenated products were formed than the amount of H2O2 consumed, suggesting that the controlled breakdown of H2O2 activates methane, which subsequently incorporates molecular oxygen through a radical process. If a source of methyl radicals can be established, then the selective oxidation of methane to methanol using molecular oxygen is possible.
Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity ( 97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH3OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at CuII oxide active centers, followed by CuI reoxidation by water with concurrent formation of hydrogen.